Pass around IndexInfo instead of bson::BSONObj

[apkar]

In the code, we pass around index information a lot, especially in the metadata handling code. The code would look much cleaner if we are passing around IndexInfo instead of bson::BSONObj. That would avoid serialization/deserialization lots of different places uses CPP typing instead of BSON type checks.

[dongxinEric]

We probably want to expand this to profile first and then audit the serialization and deserialization code in general in doc layer.